Research Inventy: International Journal Of Engineering And ScienceIssn: 2278-4721, Vol. 2, Issue 5 (February 2013), Pp 39-...
Renewable Energy Interconnection At Distribution…                                       II.       System Description      ...
Renewable Energy Interconnection At Distribution…Fig.3.Block diagram representation of grid interfacing           Fig.4.Si...
Renewable Energy Interconnection At Distribution…2.3. Fuzzy Logic Controller (Flc)         The disadvantage of PI controll...
Renewable Energy Interconnection At Distribution…using PI controller and FLC are shown in fig. 13& fig. 14. THD is less fo...
Renewable Energy Interconnection At Distribution…Fig.6.simulation results a) grid voltages, b) grid currents, c) load curr...
Renewable Energy Interconnection At Distribution… Fig.9. Simulation results a) PQ-grid, b) PQ-load,c) PQ-inv, d) dc link v...
Renewable Energy Interconnection At Distribution…Fig.12. Simulation results a) PQ-grid, b) PQ-load,c) PQ-inv, d) dc link v...
Renewable Energy Interconnection At Distribution…                    Fig.15.. FFT analysis of grid current with PI control...
Renewable Energy Interconnection At Distribution…                                                          Refernces[1].  ...
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Research Inventy : International Journal of Engineering and Science

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Research Inventy : International Journal of Engineering and Science is published by the group of young academic and industrial researchers with 12 Issues per year. It is an online as well as print version open access journal that provides rapid publication (monthly) of articles in all areas of the subject such as: civil, mechanical, chemical, electronic and computer engineering as well as production and information technology. The Journal welcomes the submission of manuscripts that meet the general criteria of significance and scientific excellence. Papers will be published by rapid process within 20 days after acceptance and peer review process takes only 7 days. All articles published in Research Inventy will be peer-reviewed.

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Research Inventy : International Journal of Engineering and Science

  1. 1. Research Inventy: International Journal Of Engineering And ScienceIssn: 2278-4721, Vol. 2, Issue 5 (February 2013), Pp 39-48Www.Researchinventy.Com Renewable Energy Interconnection at Distribution Level to Improve Power Quality 1 G.Gnaneshwar Kumar, 2 A.Ananda Kumar 1 Associate Professor, EEE Dept, KMCE&T, Devarakonda, Nalogonda (dt), Andhra Pradesh, INDIAAbstract - Electrical utilities and end users of electric power are becoming increasingly concerned aboutmeeting the growing energy demand. Maximum amount of energy demand is supplied by the non-renewablesources, but increasing air pollution, global warming concerns, diminishing fossil fuels and their increasing costhave made it look towards renewable energy sources. This paper presents a grid interfacing inverter thatcompensates power quality problems and it can also interface renewable energy sources with the electric grid.The grid interfacing inverter can effectively be utilized to perform following functions such as transfer of activepower harvested from the renewable resources, load reactive power demand support, current harmoniccompensation at PCC and current unbalance and neutral current compensation in case of 3-phase 4-wiresystem. In this paper PI controller and fuzzy logic controller are used individually for controlling the DCcapacitor voltage. The grid interface inverter configuration with IGBT is designed and the graphic models of theGrid interfacing inverter is developed using the MATLAB/SIMULINK. I. INTRODUCTION The Renewable Energy Source (RES) integrated at distribution level is termed as DistributedGeneration (DG). The utility is concerned due to the high penetration level of intermittent RES in distributionsystems as it may pose a threat to network in terms of stability, voltage regulation and Power-Quality (PQ)issues. Therefore, the DG systems are required to comply with strict technical and regulatory frameworks toensure safe, reliable and efficient operation of overall network. With the advancement in power electronics anddigital control technology [1][2], the DG systems can now be actively controlled to enhance the systemoperation with improved PQ at Point of Common Coupling (PCC). However, the extensive use of powerelectronics based equipment and non-linear loads at PCC generate harmonic currents, which may deterioratethe quality of power. Generally, current controlled voltage source inverters are used to interface the intermittentRES in distributed system. Recently, a few control strategies for grid connected inverters incorporating PQsolution have been proposed. In an inverter operates as active inductor at a certain frequency to absorb theharmonic current. But the exact calculation of network inductance in real-time is difficult and may deterioratethe control performance. A similar approach in which a shunt active filter acts as active conductance to dampout the harmonics in distribution network is proposed. A control strategy for renewable interfacing inverter based on theory is proposed. In this strategy bothload and inverter current sensing is required to compensate the load current harmonics. The non-linear loadcurrent harmonics may result in voltage harmonics and can create a serious PQ problem in the power systemnetwork [3]. Active Power Filter (APF) is extensively used to compensate the load current harmonics and loadunbalance at distribution level. This results in an additional hardware cost. Another solution is to incorporate thefeatures of APF in the, conventional inverter interfacing renewable with the grid, without any additionalhardware cost. Here, the main idea is the maximum utilization of inverter rating which is most of the timeunderutilized due to intermittent nature of RES [4]. The grid-interfacing inverter can effectively be utilized toperform functions as transfer of active power harvested from the renewable resources (wind, solar, etc.), loadreactive power demand support, current harmonics compensation at PCC , current unbalance and neutral currentcompensation in case of 3-phase 4-wire system. Moreover, with adequate control of grid-interfacing inverter, allthe four objectives can be accomplished either individually or simultaneously. The PQ constraints at the PCCcan therefore be strictly maintained within the utility standards without additional hardware cost. 39
  2. 2. Renewable Energy Interconnection At Distribution… II. System Description Fig.1. Schematic of proposed renewable based distributed generating systemIn this paper, it is shown that using an adequate control strategy, with a four-leg four-wire grid interfacinginverter, it is possible to mitigate disturbances like voltage unbalance. The topology of the investigated gridinterfacing inverter and its interconnection with the grid is presented in Fig. 1. It consists of a four-leg four-wire voltage source inverter. The voltage source inverter is a key element of a DG system as it interfaces therenewable energy source to the grid and delivers the generated power. In this type of applications, the inverteroperates as a current controlled voltage source. Fourth leg is used for neutral connection. The RES may be aDC source or an AC source with rectifier coupled to dc-link. In this paper wind energy is used as a RES, thevariable speed wind turbines generate power at variable ac voltage. Thus, the power generated from theserenewable sources needs to convert in dc before connecting on dc-link [5]–[7]. The simulink model of windfarm is given in Fig2. Wind farm generates a variable ac supply, this variable ac supply is converted into dc byconnecting a rectifier at output side. Fig.2. Simulink design of wind energy systemUnder non-sinusoidal and/or unbalanced system voltages, it is impossible to implement a shunt active filter thatsatisfies simultaneously constant real power drained from the network, sinusoidal compensated current andproportionality between the system voltage and the compensated current. This method will overcome the two ofthe three concerns mentioned above. The Clarke Transformation is no longer used and the power definitions ofthe p-q theory are not directly used. This method uses the abc-line currents, which avoids the ClarkeTransformation. The measured currents from the nonlinear load, together with a robust synchronizing circuit(PLL control circuit) form a concise controller for shunt active filter. The proposed controller forces the shuntactive filter to compensate the load current such that the current drained from the network becomes sinusoidaland balanced (contain only the fundamental positive-sequence component), even under distorted and/orunbalanced system voltage. 40
  3. 3. Renewable Energy Interconnection At Distribution…Fig.3.Block diagram representation of grid interfacing Fig.4.Simulink diagram of grid interfacingInverter inverter using fuzzy controller2.1.Control Strategy The controller requires [8] the three-phase grid current (Ia,,Ib, Ic), the three-phase voltage at the PCC(Va, Vb, Vc) and the DC-link voltage (VDC). As shown in Fig. 3, the sinusoidal waveform and the phase of thegrid current reference (Ia*, Ib*, Ic*) comes from the line voltage thanks to a PLL.The magnitude I m of the same current is obtained by passing the error signal between the DC-link voltage(VDC) and a reference voltage (VDC*) through a PI controller or fuzzy controller. Using this magnitude andphase displacement of 120° and 240° respectively, the reference three-phase grid currents ia*,ib* and ic*can be expressed as:2.2. PI controller: The actual dc link voltage is sensed and passed through a first order low pass filter (LPF) to eliminatethe presences of switching ripples on the dc link voltage and in the generated reference current signals. Thedifference of this filtered dc link voltage and reference dc link voltage (Vdc*) is given to a discrete PI regulatorto maintain a constant dc link voltage under varying generation and load conditions. The dc link voltage errorVdcerr(n) at nth sampling instant is given asThe output of discrete PI regulator at nth sampling instant is expressed as 41
  4. 4. Renewable Energy Interconnection At Distribution…2.3. Fuzzy Logic Controller (Flc) The disadvantage of PI controller is its inability to react to abrupt changes in the error signal, ε,because it is only capable of determining the instantaneous value of the error signal without considering thechange of the rise and fall of the error, which in mathematical terms is the derivative of the error denotedas ∆ε. To solve this problem, [9][10] Fuzzy logic control as it is shown in Fig 4 is proposed. T hedetermination of the output control signal, is done in an inference engine with a rule base having if-then rules in theform of "IF ε is ....... AND ∆ε is ......., THEN output is ........"With the rule base, the value of the output is changed according to the value of the error signal ε,and the rate-of- error ∆ε. The structure and determination of the rule base is done using trial-and-errormethods and is also done through experimentation. TABLE.1 FLC RULES ε NB NM NS ZE PS PM PB NB /∆ε NB NB NB NB NM NS ZE NM NB NB NM NM NS ZE PS NS NB NM NS NS ZE PS PM ZE NB NM NS ZE PS PM PB PS NM NS ZE PS PS PM PB PM NS ZE PS PM PM PB PB PB ZE PS PM PB PB PB PB2.4.Switching control As shown in Fig. 3, the hysteresis control has been used to keep the controlled current inside adefined band around the references. The status of the switches is determined according to the error. Whenthe current is increasing and the error exceeds a certain positive value, the status of the switcheschanges and the current begins to decrease until the error reaches a certain negative value. Then, theswitches status changes again. Compared with linear controllers, the non-linear ones based on hysteresisstrategies allow faster dynamic response and better robustness with respect to the variation of the non-linear load. A drawback [11] [12] of the hysteresis strategies is the switching frequency which is notconstant and can generate a large side harmonics band around the switching frequency. III. Simulation Results An extensive simulation study is carried out using MATLAB/Simulink in order to verify the proposedcontrol strategy. To achieve balanced sinusoidal grid currents at unity power factor, the 4-leg grid interfacinginverter is actively controlled under non varying renewable generating condition. The wave forms of gridvoltages, grid currents, unbalanced load current and inverter currents under absence of inverter and presence ofinverter are shown in fig.5 & fig.6. The corresponding active and reactive powers of grid (PQ grid), load (PQload) and inverter (PQ inv) under absence of inverter and presence of inverter are shown in fig.9 & fig.10.Under varying renewable generating condition , the wave forms of grid voltages, grid currents, unbalanced loadcurrent and inverter currents are shown in fig.7 & fig.8. The corresponding active and reactive powers of grid(PQ grid), load (PQ load) and inverter (PQ inv) are shown in fig.11 & fig.12.Positive values of grid active &reactive powers and inverter active & reactive powers imply that these powers flow from grid side towards PCCand from inverter towards PCC, respectively. The active and reactive powers absorbed by the load are denotedby positive signs. Before t=0.72s, the grid interfacing inverter is not connected to network, hence the gridcurrents are same as unbalanced nonlinear load currents. At t=0.72s, the grid interfacing inverter is nowconnected to network. The grid current starts changing to sinusoidal balanced from unbalanced nonlinear currentshown in fig. 7 & fig.8. At this instant active power is injecting by the inverter from RES from fig.11 & fig.12.The load power demand is less than the generated power and the additional power in fed back to the grid. Thegrid is receiving power from RES after 0.72s and it is indicated by –ve sign. At t=0.92s, generation of powerfrom RES is reduced. The active and reactive power flows between the inverter, load and grid during increaseand decrease of energy generation from RES. Total Harmonic Distortion (THD) of grid currents for 60 cycles 42
  5. 5. Renewable Energy Interconnection At Distribution…using PI controller and FLC are shown in fig. 13& fig. 14. THD is less for fuzzy logic controller its value is9.01%. THD of grid currents for single cycle using PI controller and FLC are shown in fig. 15 & fig. 16. THD isless for fuzzy logic controller compared to PI controller its value is 1.72%. By comparing all above waveformsit is clear that the fuzzy controller has high accuracy, fast response to load parameter variation and improvementin settling time. TABLE.2 SYSTEM PARAMETERS 3-phase supply (r.m.s) V= 170 v; 50 hz 3-phase non linear load R=26.66Ω; L=10mH 1-phase linear load(A_N) R=36.66Ω; L=10mH 1-phase nonlinear load (C-N) R=26.66Ω; L=10 mH Dc link capacitance & Cdc= 3000 μ F; Vdc=300v voltage Coupling inductance Lsh=20mHFig.5.simulation results a) grid voltages, b) grid currents, c) load currents, d) inverter currents under absence of inverter 43
  6. 6. Renewable Energy Interconnection At Distribution…Fig.6.simulation results a) grid voltages, b) grid currents, c) load currents, d) inverter currents under no voltage variations from RES Fig.7.simulation results a) grid voltages, b) grid currents, c) load currents, d) inverter currents using PI controller Fig.8..simulation results a) grid voltages, b) grid currents, c) load currents, d) inverter currents using fuzzy logic controller 44
  7. 7. Renewable Energy Interconnection At Distribution… Fig.9. Simulation results a) PQ-grid, b) PQ-load,c) PQ-inv, d) dc link voltage under absence of inverterFig.10. Simulation results a) PQ-grid, b) PQ-load,c) PQ-inv, d) dc link voltage under no voltage variations from RES Fig.11. Simulation results a) PQ-grid, b) PQ-load,c) PQ-inv using PI controller 45
  8. 8. Renewable Energy Interconnection At Distribution…Fig.12. Simulation results a) PQ-grid, b) PQ-load,c) PQ-inv, d) dc link voltage using fuzzy logic controller. Fig.13. FFT analysis of grid current with PI controller for 60 cycles. Fig.14.. FFT analysis of grid current with fuzzy logic controller for 60 cycles. 46
  9. 9. Renewable Energy Interconnection At Distribution… Fig.15.. FFT analysis of grid current with PI controller for single cycle. Fig.16.. FFT analysis of grid current with fuzzy logic controller for single cycle. IV. .Conclusion This papaer presents a novel method to improve the power quality at point of common coupling for a3-phase 4-wire DG system using PI controller and fuzzy logic controller for grid interfacing inverter. The gridinterfacing inverter is effectively utilized for power conditioning. This approach eliminates the additional powerconditioning equipment to improve power quality at PCC. The grid interfacing inverter with the proposedapproach can be utilized to inject real power generation from RES to the grid, and operate as a shunt ActivePower Filter (APF). The current unbalance, current harmonics and load reactive power, due to unbalanced andnon-linear load connected to the PCC, are compensated effectively such that the grid side currents are alwaysmaintained as balanced and sinusoidal at unity power factor. Moreover, the load neutral current is preventedfrom flowing into the grid side by compensating it locally from the fourth leg of inverter. When the powergenerated from RES is more than the total load power demand, the grid-interfacing inverter with the proposedcontrol approach not only fulfils the total load active and reactive power demand (with harmonic compensation)but also delivers the excess generated sinusoidal active power to the grid at unity power factor. THD of gridcurrents are decreased up to 1.72% and settling of the system is improved hence proposed fuzzy logic controller hasfast response, high accuracy of tracking the DC-voltage reference, and strong robustness to load suddenvariations. 47
  10. 10. Renewable Energy Interconnection At Distribution… Refernces[1]. S.K.Khaden, M.Basu ,M.F.Conlon ," Power quality in grid connected renewable energy systems: Role of custom power devices". ICERPQ, Granda,spain, Mar-2010.[2]. Dr.Jazeri, Dr.B.Mozafari," A Novel DG grid interface control strategy for active power injection management and power quality improvement" IPSC,Tahran,Iron, Nov-2011.[3]. A.Arulampalam, M.Barnes, A.Engler, " Control of power electronics iinterfaces in distributed generation micro grids" IJE,vol.5,2004.,page no.1-21.[4]. IonelVECHIU, GeluGURGUIATU, Emil ROSU,"Advanced Active Power Conditioner to Improve Power Quality in Microgrids" IPEC.,IEEE Conf.,2010.[5]. F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp 1398–1409, Oct. 2006.[6]. J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galván, R. C.P. Guisado, M. Á. M. Prats, J. I. León, and N. M. Alfonso, Power- electronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp.1002–1016, Aug. 2006.[7]. B. Renders, K. De Gusseme, W. R. Ryckaert, K. Stockman, L. Van- develde, and M. H. J. Bollen, “Distributed generation for mitigating voltage dips in low-voltage distribution grids,” IEEE Trans. Power. Del., vol. 23, no. 3, pp. 1581–1588, Jul. 2008.[8]. M.Singh, V.Khadkkikar, A.Chandra, Rajiv K.Varma,” Grid interconnection of renewable energy sources at distribution le vel with power quality improvement” vol.26., no.1., jan-2011.[9] Rachid Dehini, Brahim Ferdi "STATCOM Dc-bus Voltage ControllerBased on Fuzzy logic" IJAEST vol.11.,pp.281-285.[10] M.Bhanu Siva, M.R.P Reddy, Ch.Rambabu "Power Quality Improvement of Three-phase four-wire DSTATCOM with Fuzzy logic Controller" ICSIT.,vol.2,pp. 2273-2273.,2011.[11] M . Nejad, S. Pierfederici, J.P. Martin, F. Meibody-Tabar, ―Study of an hybrid current controller suitable for DC–DC or DC–AC applications,‖ IEEE Transactions on Power Electronics, vol. 22, pp 2176–2186. November 2007.[12] Copper Development Association, ―Voltage Disturbances. Standar EN50160 Voltage Characteristics in Public Distribution Systems,2004. AUTHOR’S PROFILE G.Gnaneshwar Kumar born in India. He received B.Tech degree from JNTU Hyderabad and M.Tech degree from JNTU Hyderabad. He is working as an associate professor in Khader Memorial College of Engineering & Technology at nalgonda, A.P, India. His current research interests are power systems, power systems control and automation, Electrical Machines, power systems deregulation, FACTS applications Annavarapu Ananda Kumar born in 1987, in India. He received B.Tech degree in Electrical & Electronics Engineering from Acharya Nagarjuna University, India in 2008.M.Tech degree from JNTU Kakinada. He is working as an assistant professor in Khader Memorial College of Engineering & Technology at nalgonda, A.P, India. His research interest includes Power Systems, Power System Operation & Control, and Power System Stability& Analysis. 48

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